The Book of Mormon is certainly the Keystone of our religion, and especially important to us as "Mormons." However, I don't believe this detracts from the New Testament or the Old Testament. I found a great post on the blog, Segullah, with a discussion I enjoyed reading about the relationship between the Book of Mormon and the Bible, especially the New Testament:
We celebrate having “two sticks” become one in our hands, right? The Church added the label “another witness of Christ” to the front of the Book of Mormon; wasn’t the Bible the first witness? Aren’t the Bible and the Book of Mormon a team – two witnesses that clarify what have been points of contention for centuries before the Restoration?I hope you'll check it out and let me know what you glean from it that particularly helps you understand the relationship between this diamond and pearl.